1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device which has a circuit including a thin film transistor (hereinafter referred to as a TFT). For example, the present invention relates to a method of manufacturing an electro-optical device typified by a display device which uses a liquid crystal or a self-light-emitting element.
Note that a semiconductor device in this specification refers to all devices that can function by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic appliances are all semiconductor devices.
2. Description of the Related Art
In recent years, a technique for forming a TFT with use of a semiconductor thin film (with a thickness of about several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface has become popular. TFTs have been applied to electronic devices such as ICs and electro-optical devices, and widely used particularly as switching elements of electro-optical devices for displaying images.
As the electro-optical devices for displaying images, liquid crystal display devices widely used for displays of thin television sets, personal computers, cellular phones, and the like are known. Many liquid crystal display devices are of active matrix type because high-definition images can be obtained as compared with those of passive matrix type. An active matrix liquid crystal display device includes a pixel portion serving as a display region, a terminal portion manufactured at an end of a substrate for electric connection with a wiring of a circuit provided over another substrate, and the like. In this liquid crystal display device, gate wirings and source wirings extend in matrix and pixel electrodes disposed in the matrix are controlled by TFTs, whereby images are displayed. Specifically, by voltage applied between a selected pixel electrode and a counter electrode corresponding to the pixel electrode, a liquid crystal layer disposed between the pixel electrode and the counter electrode is optically modulated. Observers recognize this optical modulation as a display pattern.
The application range of the electro-optical devices typified by such active matrix liquid crystal display devices is expanding, and demands for higher definition, a higher aperture ratio, and higher reliability are increasing as a screen size gets larger. At the same time, there are increasing demands for higher productivity and cost reduction.
Conventionally, an amorphous silicon film has been preferably used as an amorphous semiconductor film because an amorphous silicon film can be formed over a large-area substrate at low temperatures of 300° C. or lower. Moreover, many inverted staggered (or bottom gate) TFTs whose channel formation regions are formed of amorphous semiconductor films have been used; therefore, a standard manufacturing method thereof has been established. For example, Japanese Published Patent Application No. H9-171197 has disclosed a method of forming a TFT portion and a terminal portion (pad portion) over a substrate with use of five photomasks (five photographic stages).